INTERNAL REVERSIBLE HYDROGEN EMBRITTLEMENT AND HYDROGEN GAS EMBRITTLEMENT OF AUSTENITIC STAINLESS STEELS BASED ON TYPE 316

The internal reversible hydrogen embrittlement (IRHE) of austenitic Fe(10-20)Ni17Cr2Mo alloys based on type 316 stainless steel was investigated by tests using the slow strain rate technique from 80 to 300 K in comparison with its effect on the hydrogen gas embrittlement (HGE) of the alloys in hydrogen at a pressure of 1 MPa. The IRHE and HOE of the alloys in 70 MPa hydrogen at room temperature was also investigated. At low temperatures, IRHE occurred below a Ni content of 15% (Ni equivalent (Ni-eq):29%), increased with decreasing temperature, reached a maximum at 200 K, and decreased with further decreasing temperature, similarly to the temperature dependence of HOE. At room temperature, IRHE and HOE were observed below a Ni content of 14% (Ni-eq:28%) and decreased with increasing Ni content (Ni-eq). The dependence of HGE on hydrogen pressure increased with decreasing Ni content (Ni-eq). Hydrogen-induced fracture closely related to the strain-induced alpha' martensite structure and twin boundaries mainly occurred for both IRHE and HGE. Dimple ruptures caused by hydrogen segregation occurred in only IRHE at 150 K. The content of strain-induced alpha' martensite increased with decreasing temperature and Ni content (Ni-eq). Thus, the susceptibility to IRHE and HOE depended on Ni content (Ni-eq). It was concluded that both IRHE and HOE were controlled by the amount of strain-induced alpha' martensite above 200 K, whereas they were controlled by the hydrogen transport below 200 K.